1. Field of the Invention
This invention relates to a variable magnification lens which is capable of changing an imaging magnification in accordance with the purpose of use and an optical system which is capable of photographing an image recorded by a film at a magnification most suitable for the film, and to an optical apparatus, such as an image transforming apparatus, using this optical system.
2. Description of Related Art
Imaging optical systems which are designed to be both-side telecentric and to change the imaging magnification have been proposed, for example, by Japanese Patent Kokai No. 2001-27726 and Japanese Patent No. 2731481.
The optical system set forth in Kokai No. 2001-27726 includes, in order from the object side, the first lens unit with positive refractive power, the second lens unit with positive refractive power, the third lens unit with negative refractive power, and is the fourth lens unit with positive refractive power. This optical system is thus constructed to be both-side telecentric and to change the imaging magnification.
The optical system set forth in U.S. Pat. No. 2,731,481 includes, in order from the object side, the first lens unit with positive refractive power, the second lens unit with negative refractive power, and the third lens unit with positive refractive power. This optical system is thus constructed to be both-side telecentric and to change the imaging magnification while keeping an object-to-image distance constant.
The imaging optical system of the present invention includes a variable magnification optical system comprising, in order from the object side toward the image side, a first lens unit with positive refractive power, a second lens unit with positive refractive power, a third lens unit with negative refractive power, a fourth lens unit with positive refractive power, and an aperture stop interposed between the third lens unit and the fourth lens unit. The variable magnification optical system changes an imaging magnification while keeping an object-to-image distance constant. The imaging magnification is changed by varying spacing between the first lens unit and the second lens unit, spacing between the second lens unit and the third lens unit, and spacing between the third lens unit and the fourth lens unit. When the imaging magnification is changed, the imaging optical system satisfies the following conditions in at least one variable magnification state:
|En|/L greater than 0.4
|Ex|/|L/xcex2| greater than 0.4
where En is a distance from a first lens surface on the object side of the variable magnification optical system to the entrance pupil of the imaging optical system, L is the object-to-image distance of the imaging optical system, Ex is a distance from the most image-side lens surface of the variable magnification optical system to the exit pupil of the imaging optical system, and xcex2 is the magnification of the entire system of the imaging optical system.
The imaging optical system of the present invention preferably satisfies the following conditions:
1.0 less than MAXFNO less than 8.0
|xcex94FNO/xcex94xcex2| less than 5
where MAXFNO is the smallest object-side F-number where the imaging magnification of the imaging optical system is changed, xcex94FNO is a difference between the object-side F-number at the minimum magnification and the object-side F-number at the maximum magnification in the entire system of the imaging optical system, and xcex94xcex2 is a difference between the minimum magnification and the maximum magnification in the entire system of the imaging optical system.
The imaging optical system of the present invention preferably satisfies the following condition:
0.6 less than |(R3f+R3b)/(R3fxe2x88x92R3b)| less than 5.0
where R3f is the radius of curvature of the most object-side surface of the third lens unit and R3b is the radius of curvature of the most image-side surface of the third lens unit.
The imaging optical system of the present invention is preferably constructed so that the most object-side lens of the first lens unit has positive refractive power.
The imaging optical system of the present invention is preferably constructed so that the first lens unit includes, in order from the object side, a lens with positive refractive power, a lens with negative refractive power, and a lens with positive refractive power.
The imaging optical system of the present invention is preferably constructed so that the third lens unit includes at least two meniscus lenses, each with a convex surface directed toward the object side.
The imaging optical system of the present invention is preferably constructed so that the third lens unit includes two meniscus lenses, each with negative refractive power, and one meniscus lens with positive refractive power.
In the present invention, an optical apparatus using the imaging optical system of the present invention is provided.
According to the present invention, the imaging optical system in which even when the imaging magnification is changed, the object-to-image distance remains unchanged and the fluctuation of the F-number is minimized, and the optical apparatus using the imaging optical system can be provided.
These and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.